1. Field of the Invention
The present invention concerns a method for magnetic resonance (MR) imaging in which a spatially resolved quantification of the T1 relaxation time is created with a gradient echo method. The invention furthermore concerns a magnetic resonance system to implement such a method.
2. Description of the Prior Art
Magnetic resonance tomography (MRT) is an examination modality with which regions inside an examination subject can be shown with high resolution and good contrast, in particular even soft tissues such as muscles and organs. A dependency of the measured MR signal on the T1 and T2 relaxation times can be achieved by appropriate acquisition sequences. The relaxation times T1 and T2 are the time constants of the decay of the macroscopic magnetization excited in the tissue, which vary for different tissues. The spin lattice relaxation time T1 is characteristic of a process that produces the reestablishment of the longitudinal steady state magnetization that appears in an applied basic magnetic field B0.
For example, conventional methods use T1-weighted “delay enhancement” methods in order to characterize the tissue state of the cardiac muscle (myocardium). Such methods provide a high tissue contrast and are presently the standard in the determination of the local vitality. However, these methods are not quantitative, meaning that they merely enable a delimitation between healthy and destroyed tissue but not a differentiation in the transition range or in ischemic but not infracted tissue regions.
Such a quantification could be achieved with a quantitative, per pixel representation of the T1 time since this is a measurement-independent variable and enables the subdivision of the tissue into regions with varying T1 values. However, conventional methods for T1 quantification can not be used to depict the cardiac muscle because heart beat and breathing movement prevent an artifact-free depiction.
The MOLLI (Modified Look-Locker Inversion Recovery) method is known in order to quantify the T1 relaxation time in the myocardium. The method is based on an inversion recovery method in which the steady state magnetization is inverted by means of a 180° pulse. Measurements along the T1 decay curve are then conducted with subsequent readout steps. The MOLLI method has the advantage that all necessary data can be acquired within a breath hold phase in order to generate a T1 map for an imaging slice. Due to the properties of the single shot readout steps that are used, the method is subject to limitations with regard to the possible temporal and spatial resolution. In particular, the method enables the acquisition of only one slice so that a volumetric coverage of the heart during a breath hold phase is not possible with the MOLLI acquisition scheme. The measurement of an additional slice in a second breath hold phase is advantageous since in the meantime a movement of the examination subject has most likely occurred, such that sequentially acquired slices exhibit movement artifacts and cannot be directly associated with one another for volumetric presentation.